This invention relates to a method of preparing thin film by CVD (chemical vapor deposition), particularly to a method of subliming a solid material to form a substrate with a thin film including a component useful in a ferroelectric material, an electrode material, an LSI (large-scale integrated circuit) wiring material, an optical element, a superconducting material or the like, more particularly to a method of subliming a material which enables formation of a uniform film without variation in film growth rate or film composition over time.
Methods of preparing single crystal film and polycrystalline film fall in two categories: the dry process and the wet process. The dry processes has been widely adopted because the quality of the film grown by this process is generally better than that produced by the wet process.
Dry process methods include physical film forming methods such as vacuum deposition and ion plating, and chemical film forming methods such as chemical vapor deposition (CVD). Among these, CVD has been widely adopted because it enables the film growth rate to be controlled with ease, does not require the film growth to be conducted under a high vacuum, is capable of high-speed film growth and is otherwise suitable for volume production.
CVD uses a metal complex as precursor. Thermochemical vapor deposition, photochemical vapor deposition, plasma CVD and the like are adopted to decompose the precursor vapor during growth of a metal thin film. The complex compound precursor is generally a metal organic .beta.-diketonate complex whose ligand is dipivaloylmethane, hexafluoroacetylacetone or the like.
When a conventional metal organic .beta.-diketonate complex is used in CVD, the high melting point of the complex requires it to be vaporized by sublimation from solid state. Since the volume of the solid material in the precursor vessel therefore varies as the sublimation proceeds, the vaporization rate varies.
Since the amount of vaporized material therefore generally decreases with passage of time, it is difficult to secure a constant film growth rate over long periods. When it is attempted to grow a compound thin film including two or more metal elements, moreover, the film composition is difficult to control owing to the change in amount of vapor with passage of time.
While these problems could be overcome by using a material with a low melting point and vaporizing it from liquid state, few metal precursor meet this description. Ordinary precursor therefore have to be sublimed from solid state.
JP-A-4-333572 teaches a method of bringing a carrier gas into contact with a porous body whose surface and/or interior is charged with a metal organic complex as the precursor. This publication claims that this method increases the evaporation rate and enables a large amount of the metal organic complex to be stably transported to the film growth site. In fact, however, when the precursor is retained in a porous body, the area of the exposed precursor remains constant only for a short period at the start, and then, as the evaporation proceeds, unavoidably begins to vary owing to progressive exposure of the precursor retained in the countless pores at the surface of the porous body. The evaporation rate therefore varies.
The object of this invention is therefore to provide a method which suppresses time-course variation of evaporation rate during sublimation of a solid-state precursor to enable growth of film by CVD under a stable evaporation rate.
The above and other objects and features of the present invention will become apparent from the following description made with reference to the drawings.